There are many instances when combining two or more antibiotics is necessary
or desirable in treating an infection or disease. This may be the case when there is a
mixed infection, in which there may be several organisms, or when there is a severe or
life threatening infection of unknown origin. By combining several agents, they can
sometimes be made more effective than either agent alone (synergy), thus allowing
lower doses and reducing complications of therapy. With certain organisms, the use of
combination therapy is necessary to prevent rapid resistance to the drug that is used.
Chemoprophylaxis is the use of antibiotics to prevent infection in a healthy
individual or to prevent infection by other organisms (called superinfection) in an
individual already being treated for an infection. This generally involves giving low
doses of a drug on an infrequent schedule. For example, individuals who have had
rheumatic fever as children may take a sulfonamide once a day (instead of the four
times daily needed for treating an infection) to prevent the development of bacterial
endocarditis, a life-threatening infection involving the heart.
Superinfections may occur during the use of anti-infective therapy. This is simply
the overgrowth of nonsusceptible normal body flora. There is always an alteration of
body flora, particularly that found in the GIjettie@cyberback.com , urinary, and respiratory
tract during antibiotic therapy. Usually, the organism that tends to overgrow is Pseudomonas,
Candida, fungi, or beta-lactamase producing staphylococci. Superinfection is most
noticeable when broad-spectrum antibiotics have been administered over a period of 14
days or more.
a. Natural Penicillin.
(1) Natural penicillins are derived from certain species of molds and other
fungi. They produce their effects by inhibiting biosynthesis of cell wall mucopeptides.
All classes of penicillin are bactericidal.
(2) Oral absorption of natural penicillin is incomplete and variable, except for
the newer penicillin VK products. Oral absorption becomes more predictable if the
penicillin is taken on an empty stomach, 1 hour before or 2 hours after meals. IV and
IM routes produce transient, high-blood levels of the antibiotic. Penicillin is widely
distributed to all tissues, especially soft tissues. However, it is not distributed to ocular,
skeletal and cardiac muscle and to cerebrospinal fluid unless inflammation is present.